<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    #box{
      width: 320px;
      height: 200px;
      position: fixed;
      left: calc(50% - 160px);
    }
    .textBox{
      position: absolute;
      top: 0;
      left: 0;
      width: 320px;
      height: 200px;
      font-size: 24px;
      font-weight: bold;
      text-align: center;
      line-height: 200px;
      z-index: -1;
    }
  </style>
</head>
<body>
  <div id="box">
    <div class="textBox">谢谢惠顾</div>
    <canvas id="canvasBox"></canvas>
  </div>

  <script>
    const box = document.getElementById('box')
    const canvas = document.getElementById('canvasBox')
    canvas.width = 320
    canvas.height = 200
    const ctx = canvas.getContext('2d')  // 创建画布
    ctx.fillStyle = 'transparent' // 底色
    ctx.fillRect(0, 0, 320, 200)
    ctx.fillStyle = 'grey'  // 准备填充色
    ctx.fillRect(0, 0, 320, 200)
    ctx.globalCompositeOperation = 'destination-out'



    // 鼠标摁住不放，在 box 上移动时，擦除颜色
    let flag = 0
    // 监听鼠标摁下
    canvas.addEventListener('mousedown', () => {
      flag = 1
    })
    // 监听鼠标放开
    canvas.addEventListener('mouseup', () => {
      flag = 0
    })

    // 鼠标移动
    // 当用户鼠标一直是摁住的状态
    canvas.addEventListener('mousemove', (e) => {
      e.preventDefault()
      if (flag === 1) {
        // 擦除
        // 准备一个正方形
        // 获取到鼠标摁下那一刻的坐标
        const x = e.x - box.offsetLeft;
        const y = e.y - box.offsetTop;

        ctx.beginPath(); // 落笔
        ctx.rect(x, y, 20, 20)
        ctx.fill()
      }
    })



    // 控制textBox容器中的文本内容是随机的
    
  </script>
</body>
</html>